Metallic components for use in corrosive environments and method of manufacturing

ABSTRACT

The present invention relates to a metallic component and method of manufacture of the component for use in a corrosive environment, such as components used in fossil fuel recovery or used in chemical facilities. The components comprise at least one metallic portion having a deep, stable layer of compressive stress for providing life extension and mitigation of fatigue and corrosion related failures. Preferably, the layer of compressive stress has a depth that exceeds the depth of any surface irregularities.

This application claims the benefit of U.S. Provisional Application No.61/340,282, filed Mar. 15, 2010.

BACKGROUND OF THE INVENTION

The present invention relates generally to metallic components used in acorrosive environment, such as components used for fossil fuel recoverythat have improved properties and methods of manufacturing suchcomponents. More specifically, the present invention are new and novelmetallic components for use in corrosive environments such as componentsused for fossil fuel recovery, and methods of manufacture, whereby thecomponents have improved properties for mitigating or preventing thedeleterious effects of stress corrosion cracking (SCC) and fatigue onthe useful life of the metallic components. Such components aretypically used in recovery and distribution of fossil fuels or used inchemical plant applications.

In the recovery and distribution of fossil fuels and the operation ofpetrochemical refineries and other types of chemical plants, failure ofmetallic components is often a result of the combination of stress aswell as one or more corrosive elements, such as hydrogen sulfide, H₂S,ammonia, or chlorine, to which the component is exposed in service. Theelevated temperatures, pressures, and applied stresses, either static oralternating, to which such components are exposed, contribute to theirdegradation and the rate at which corrosive related failure processesoccur, especially SCC and corrosion fatigue. The life of metalliccomponents in these environments and applications is often limited, andpremature component failures restrict production and increaseoperational costs.

In oil and natural gas well drilling to recover fossil fuels, the depthto which drilling can be performed is limited by the materials availablefor the drill pipe, tubing, and casing, generally referred to as OilCountry Tubular Goods (OCTG). In offshore drilling, the materialstrength of the OCTG and drill components limits the depth of water andthus the distance from shore that is accessible. The pipe used fordistribution of the fossil fuel is generally known as Line Pipe TubularProducts (LPTP). As used herein, OCTG, LPTP and drill and drill rigproducts and components will collectively be referred to as FuelRecovery Components. Line Pipe Tubular Products may be fabricated aseither seamless or welded, with the seamless generally used for the mostdemanding applications. It is well known that the strength of FuelRecovery Components formed from metallic materials, such as steelalloys, can be increased by various heat treatment procedures, and avariety of heat treatable Fuel Recovery Components are available havinga range of strengths (including the yield, ultimate, and fatiguestrengths). The toughness of the alloy in stress corrosion cracking,given by the parameter K_(ISCC), is a measure of the resistance tocracking, and generally is reduced as the yield and ultimate strengthare increased by heat treatment. Therefore, the strength of metallicmaterial that can be used in a SCC prone environment, such asenvironments that Fuel Recovery Components often operate in, is limited.

The weight of various Fuel Recovery Components, such as pipe hangingfrom a drill platform in well drilling operations, is often a primarysource of applied stress. In horizontal drilling, now used for both oiland gas machinery with minimal environmental impact, a significantbending stress is applied to the Fuel Recovery Components (such as apipe, tubing, casing, or coupling) in the transition from vertical tohorizontal drilling. Higher strength steel drill pipes, distributionpipes, and casing allow deeper wells and drilling in deeper water,providing a major economic advantage in tapping deep oil and gas fieldsboth on land and off-shore. However, when the oil or gas wells are“sour” with H₂S present, or when the stressed pipe is exposed toseawater during offshore drilling or subsurface salt-water depositscontaining dissolved chlorine, the pipe and casing are subject to SCC.It is then a common practice to limit the strength of steel pipe,tubing, casing and other Fuel Recovery Components because the softer,weaker material is not susceptible to SCC. Expensive SCC “down hole”failures are avoided at the cost of limiting the possible range ofdrilling.

The failure of Fuel Recovery Components as well as components used in awide variety of chemical plant applications generally results in majoreconomic loss, if not catastrophic damage impacting public safety. It iswell known that failure of such metallic components is most commonlycaused by the mechanisms of SCC or fatigue. SCC occurs from the surfacebeing exposed to a corrosive media to which the alloy is susceptibleunder a primarily constant steady state applied tensile stress exceedinga tensile stress threshold specific to the alloy and the environment.Fatigue failures occur under the influence of alternating appliedstress, generally accompanied by a steady mean stress, and oftenoriginate from a surface flaw such as a corrosion pit, SCC, or scratch.Corrosion fatigue is a combination of fatigue failure in the presence ofa corrosive environment, in effect adding a SCC component to failureunder cyclic loading.

There are several current chemically-based practices that are used toattempt to prevent or reduce SCC and corrosion fatigue failure inmetallic components that are used in fossil fuel and chemicalapplications:

-   -   1) use alloys with enhanced corrosion resistance such as        stainless steels;    -   2) use sacrificial anodes to cathodically protect the metallic        components;    -   3) chemically alter the environment of the component with        alkaline substances or other protective fluids;    -   4) paint, plate, or coat components to shield the metallic        surfaces from the corrosive environment; and    -   5) limit the strength of the steels used and applied stress        levels.

All of these chemistry or coating-based methods have limitations, orhave shown limited improvement in performance at relatively highimplementation costs to the end-user. Components formed from corrosionresistant alloys are relatively expensive and often are not costeffective. Cathodic protection offers only temporary benefit byredirecting the corrosion process to the sacrificial material until itis consumed. Adding chemicals to neutralize corrosive elements, oftenknown as ‘down-hole’ injection, also offers only a temporary solutionbecause the chemicals will eventually diffuse away or be consumed inreaction. Paint and coatings will peal, wear away, or be scraped offeventually exposing the surface to the corrosive environment, andgenerally cannot be renewed on the casing and components installed downin the well. Reducing the strength of the material and designing forlower applied stresses provides a long-term solution but limitsperformance, as noted above.

Mechanical methods have been used or proposed that are designed to placethe surface layer that will be in contact with the corrosive media in astate of residual compression in an attempt to mitigate either SCC orfatigue. Such methods include shot peening, laser shock processing(LSP), low plasticity burnishing (LPB), and deep rolling.

Shot peening has been widely used in many industries for decades tointroduce a relatively shallow surface layer (<0.5 mm) of residualcompression in metallic components, and has been used to reduce thesusceptibility of such components to SCC. However, because it is arandom impact process, shot peening severely cold works the surface inorder to cover the surface with impact dimples and produce thecompressive layer. The beneficial compressive residual stresses in thehighly cold worked surface are then known to be susceptible to rapidthermal relaxation at relatively lower service temperatures and istherefore unacceptable for certain components. Further, the relativelyshallow cold worked residual compression layer is also susceptible toloss of compression by tensile overload in work hardening materials,again making the method unacceptable for certain components. Shotpeening also produces a roughened dimpled surface that makes it moredifficult to detect a crack or flaw using nondestructive inspection(NDI) methods such as ultrasonic and eddy current means, again making itunacceptable for certain applications.

LSP has been proposed for oil, gas, and petrochemical weld applications.LSP can produce relatively deep (˜1 mm) compression, but isprohibitively expensive for components having large surface areasneeding treatment. Further, LSP requires an ablative coating to beapplied along the surface of the component being treated that generatespost-processing debris that must be removed, thereby adding additionalcost. In addition, LSP requires repeated shocking cycles to achieve a 1mm deep compressive layer, thereby adding additional cost and processtime. Also, LSP has been known to damage the surface in three ways thathave been shown to contribute to component failure. First, internalcracking can occur due to superposition of echoing shock waves. Second,LSP shock waves are known to cause twinning in some crystals, like intitanium alloys, that are associated with subsequent fatigue crackinitiation. Third, LSP is known to produce laser burns and local areasof residual tension that occur if the ablative coating is breached sothat the laser strikes the bare metal surface; surface tension from suchburns will exacerbate SCC. Accordingly, components treated using LSPoften require post-processing inspection that can significantly increasecost and processing time. Components that have been damaged by the LSPprocess often require additional processing or must be scrapped, therebyfurther increasing cost and processing time. Further, denting of thesurface at each shock point by LSP may also require refinishingoperations. Like shot peening, the dented surface reduces theeffectiveness of eddy current and ultrasonic NDI techniques that arevital to monitor the integrity of critical components.

Deep rolling, a form of roller or ball burnishing, has also beenproposed to introduce a layer of compression and cold work deeper thanthat provided by shot peening and improves the surface finish of thetreated area. Deep rolling creates a highly cold worked surface in thearea being treated in order to mechanically strengthen the surfacematerial while introducing compression. The depth and magnitude of coldwork along the surface typically exceeds that produced by shot peening.Cold working, however, is well known to increase the susceptibility ofmetals to SCC. Annealing or tempering to reduce or eliminate cold workand reduce hardness is a common remedy used to reduce suchsusceptibility to stress corrosion cracking. However, such processesincrease processing time and cost and may be difficult to perform oncertain components. Accordingly, deep rolling suffers from theconflicting influences of the detrimentally increased cold working ofthe surface as the beneficial residual compression is introduced.

Both SCC and fatigue failures are well known to initiate from very smallsurface irregularities such as surface cracks, small crevices, flaws,scratches, persistent slip bands, even crystal twin boundaries createdby deformation, and the like. Such surface irregularities are known toserve as sites of increased ion concentration, exacerbating SCC. Thesurface irregularities are also points of stress concentration that arewell known to serve as fatigue crack initiation sites. Cold working,such as by shot peening and deep rolling, is known to damage thecrystalline structure, creating slip bands, dislocations, and twinningthat make the surface more susceptible to chemical attack. Workhardening, like hardening by heat treatment, makes metals moresusceptible to SCC. Further, deforming the surface to introduce residualcompression in ways that increase the surface irregularities, such as byshot peening and LSP, is also known to create local sites for SCC andfatigue initiation.

Accordingly, a need exists for corrosive resistant components, such asFuel Recovery Components as well as components for use in a wide varietyof chemical plant applications where SCC failures occur, that haveimproved properties for mitigating or preventing the deleterious effectsof SCC and fatigue on useful life. A practical, inexpensive method isneeded for introducing a relatively deep, stable layer of beneficialcompressive stress along and into the surface of such components thatprotects against or reduces SCC, fatigue, corrosion fatigue and relatedfailure modes, and provides an improved surface finish with low coldworking, so that the metallic materials forming the components can beused at their full available strength.

SUMMARY OF THE INVENTION

The present invention relates generally to corrosive resistantcomponents, such as Fuel Recovery Components as well as components usedin a wide variety of chemical plant applications, and their method ofmanufacture. Such corrosive resistant components have improved stresscorrosion and fatigue properties for mitigating or preventing thedeleterious effects of SCC and fatigue on useful life of the metalliccomponents.

The preferred method of the invention disclosed herein dramaticallyimproves the SCC, corrosion fatigue, and general fatigue performance ofmetallic components used in a wide variety of applications, such asfossil fuel recovery and chemical plant applications, manufactured fromtraditional low-cost alloys, such as carbon steel, without alteringeither the alloy chemical composition or the geometry of the component.The invention puts the surface of the metallic component that is incontact with the corrosive environment, and the layer of materialimmediately below the surface, into a state of high residual compressionwith controlled low cold working to a sufficient depth to encompass thesurface irregularities. In a preferred embodiment, the componentsinclude tubular products such as pipe, tubing, casing, and couplings,having the outside, or inside, or both surfaces processed using variousmachine tools or robots commonly available that can be used to positionand move the burnishing tools to cover all or a portion of the surfacebeing treated. In a preferred embodiment of the invention, a preferredmethod includes a surface treatment which is performed in a singleautomated operation during initial manufacture or during repair andoverhaul of existing components.

In a preferred embodiment of the invention, a layer of compression iscreated using one or more ball or roller burnishing tools and normalforces and tool positioning that produce a relatively uniform layer ofcompression extending to a depth of about 1 mm or more, so that thesurface being treated when in contact with a corrosive media and anysurface irregularities, such as discussed above, are confined in a layerof compressive residual stress. The magnitude of the residualcompression is generally on the order of the yield strength of the alloyso that the surface layer remains in compression under any appliedtensile stresses experienced by the component during service, and thestress at the surface in contact with the corrosive environment neverexceeds the critical tensile threshold for SCC. In a preferredembodiment, scratches and other surface irregularities are maintained incompression, even under external applied tensile loading during service,thereby fatigue initiation is prevented or significantly reduced.

In another preferred embodiment of the invention, the position and forceapplied to one or more tools during the burnishing process is controlledto develop a specified level of low cold work while introducing a layerhaving a desired magnitude of compression. The layer of compression isof a magnitude and depth such that the sum of residual and appliedstress at the surface and to a depth of at least nominally about 0.5 mmnever exceeds the threshold for SCC in the specific corrosiveenvironment of the application or the fatigue endurance limit of thematerial. In a preferred embodiment, the depth of compression is chosenso that the all or a majority of surface irregularities that may operateas sites of crack initiation are confined within the depth of thecompressive layer.

Processing by the method of this invention allows inexpensive steel oralloy to be used for components that operate in a corrosive environment,such as Fuel Recovery Components as well as components used in a widevariety of chemical plant applications (such as piping, casing,couplings and related components), that are normally restricted to useonly in applications not subject to SCC, to then be placed in service incorrosive environments, such as in “sour” wells, and applicationspreviously requiring more costly alloys, such as stainless steels.Inexpensive steels processed by the method of this invention can then beused at their optimum temper and strength to allow higher appliedstresses in service allowing drilling of deeper wells at lower cost.

In a preferred embodiment, the surface finish is improved by burnishingwith a finely finished tool to both reduce surface irregularities whileenhancing the detection limits of NDI. Improved NDI detection limitsreduce inspection costs and allow more reliable detection of flaws.Elimination or the reduction of surface irregularities improves both SCCand fatigue resistance, as noted above.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the invention will be bestunderstood with reference to the following detailed description of aspecific embodiment of the invention when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a plot of the subsurface residual stress and diffraction peakwidth distributions produced by the burnishing method of the subjectinvention in low cost P110 casing coupling material. The residual stressshown is additive to the applied stress in service;

FIG. 2 is a bar chart comparing the surface roughness measured on thesurface of P110 steel coupling stock as-manufactured versusafter-processing showing the improved finish with the method of thecurrent invention;

FIG. 3 is a bar chart showing failure by SCC in the National Associationof Corrosion Engineers' (NACE) 1% H₂S solution of as-manufactured P110coupling stock samples after only 10 hour exposure, and showing thatexposure for over 420 hours did not break the same P110 material afterprocessing with the method of the invention;

FIG. 4 is a schematic illustration showing the relationship between theburnishing apparatus and control system for properly inducing a desiredstress distribution along and into the surface of a component;

FIG. 5 is a flow diagram illustrating a preferred method of the subjectapplication;

FIG. 6 is a schematic illustration of a portion of a component beingmanufactured using the method of the subject invention; and

FIG. 7 is a schematic illustration of the surface of the component shownin FIG. 6 illustrating various surface irregularities.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the “as-received” subsurface residual stressdistributions created by the current methods of manufacturing componentsused in a corrosive environment, such as Fuel Recovery Components aswell as components used in a wide variety of chemical plant applications(including, but not limited to pipe, tubing, casing and OCTG), is shownin comparison to the beneficial high magnitude deeper compressiveresidual stress layer created by a preferred method of the currentinvention. The residual stress is shown in both units of the commonengineering usage in the United States, where 1 ksi=1000 psi, and in SIunits of MPa. Prior art manufacturing methods used for such typicalcomponents, such as OCTG products, provide only relatively shallowcompression, generally less than −30×10³ psi (−30 ksi or −200 MPa)extending to a depth of only about 0.020 in. (0.5 mm), such as in theexample shown. The prior art practice does not attempt to control oroptimize in any way the state of residual stress on the surface of theproducts. The deeper and higher magnitude compressive residual stressdistribution produced by the method of the present invention is shown as“LPB” in FIG. 1. The method of the invention introduces compression ofmuch higher magnitude, nominally −100 ksi (−700 MPa), approaching theyield strength of the material and extending to a depth greater thanabout 0.040 in. (1 mm). In a preferred embodiment of the invention, thedepth of compression produced by the method of the subject inventionexceeds the depth of any surface irregularities.

Referring to FIG. 3, SCC of P110 casing material loaded as U-bendsamples in tension in a “sour” H₂S solution is shown as having beeneliminated after processing by the method of the subject invention.As-manufactured casing material failed in only 10 hours. In a preferredembodiment of the present invention, SCC and failure from fatigue orcorrosion fatigue damage is mitigated by introducing a layer ofcompressive residual stress using a process of LPB. Inducing acompressive stress distribution along a surface by LPB is shown anddescribed in U.S. Pat. Nos. 5,826,543 and 6,415,486, which areincorporated herein by reference.

Referring to the bottom of FIG. 1, the method of the invention creates adesired compressive stress distribution by deforming the material aminimal amount to achieve the required compression. LPB produces lessthan approximately 5% cold work while creating a depth and magnitude ofcompression comparable to LSP or deep rolling. It has been found thatusing LPB for components used in a corrosive environment, such as FuelRecovery Components as well as components used in a wide variety ofchemical plant applications, provides the desired compressive stressdistribution along and into the surface being treated without theincreased susceptibility to SCC and corrosion fatigue caused by coldworking of the surface such as by shot peening or deep rolling andwithout the detrimental effects often caused by laser shock peening.

Referring to FIG. 2, processing by the method of the invention has beenshown to improve the surface finish from about 204 to about 90micro-inches, reducing the roughness of the surface being treated. Inthe preferred embodiment, the method of the subject invention improvesthe surface finish by rolling a hardened ball or roller having agenerally smooth surface along the surface of the component. The surfaceproduced by the subject invention depends upon the burnishing parametersselected, such as the smoothness of the hardened ball or roller, theball or roller diameter, and the force with which it is pressed againstthe surface of the piping or other component. In a preferred embodiment,the selected burnishing parameters are selected to produce a smoothsurface effective for improving detection limits for NDI and eliminationor reduction of surface irregularities that can become fatigue crack orcorrosion pit initiation sites.

In another preferred embodiment, as illustrated in FIG. 4, the methoduses a burnishing apparatus 100 having a constant volume flow of fluid102 to support a hydrostatic burnishing member 104 (such as shown ortaught in U.S. Pat. No. 6,415,486 which is incorporated herein byreference) that rolls along a surface portion 106 of the component 108being treated with sufficient force to induce compressive stress 110having a desired magnitude and depth of compression and also allowslarge surface areas of components, such as Fuel Recovery Components aswell as components used in a wide variety of chemical plantapplications, to be processed rapidly with minimum down time and toolcosts.

Another embodiment of the method of the subject invention, as shown inFIG. 4, a computer numerically controlled (CNC) apparatus 112 is used toposition one or more burnishing members 104 of a burnishing apparatus100 to guide the members 104 in a predetermined pattern along thesurface portion 106 of the component 108 being treated with sufficient,but not necessarily constant pressure, to create a desired distributionof compressive residual stress 110 on and into the surface portion 106of the component 108.

A further embodiment of the method utilizes a means of rotating thecomponent 114 being processed in the manner of a lathe or similar means,while one or more burnishing apparatuses are held at fixed angularpositions and are positioned down the length of the rotating componentin a helical pattern to cover at least a portion of the outside orinside surface.

Referring to FIG. 5, a preferred embodiment of the method of the presentinvention is shown whereby components expected to operate in a corrosiveenvironment are identified (step 200). One or more surface portions ofone or more of the identified components are identified and selected forreceiving a surface treatment (step 202), such as burnishing. Theenvironment that the surface portions will be exposed to, as well asvarious operating applied, static, and alternating stresses expected tobe encountered, are identified (step 204). A stress distribution foreach surface portion is then determined based upon the geometry of thecomponent; the material forming the component along the surface portionbeing treated; the environment to which the component will be exposed;the use of the component; and the temperatures, pressures, and applied,static, and alternating stresses to which the component is expected tobe exposed during service (step 206). Burnishing parameters, such as thesmoothness of the burnishing member, the diameter of the burnishingmember, the force with which the burnishing member is pressed againstthe surface being treated, and the pattern of burnishing are thendetermined based on the desired stress distribution (step 208). In apreferred embodiment of the invention, the burnishing parameters areselected such that a relatively uniform layer of compression is inducedalong the surface portion and extending to a depth sufficient such thatthe surface being treated when in contact with a corrosive media and anysurface irregularities are confined in a layer of compressive residualstress. In another preferred embodiment of the invention, the burnishingparameters are also selected such that the magnitude of the residualcompression induced along and into the surface portion is generally onthe order of the yield strength of the alloy forming the surface portionof the component so that the surface layer remains in compression underany applied tensile stresses expected to be experienced by the componentduring service. In another preferred embodiment of the invention, theburnishing parameters are selected such that in operation of thecomponent, the stress at the surface in contact with the corrosiveenvironment never exceeds the critical tensile threshold for SCC. Inanother preferred embodiment, the burnishing parameters are selectedsuch that surface irregularities along the surface portion aremaintained in compression, even under external applied tensile loadingduring service of the component, thereby reducing or eliminating fatigueinitiation. The method further comprises the step of performing aburnishing operation along the one or more of the identified surfaceportions using the determined burnishing parameters to induce thedesired stress distribution along and into the identified surfaceportions (step 210). In a preferred embodiment, the burnishing operationis performed such that the amount of cold work induced along the surfaceportions is less than about 5%. In a preferred embodiment, theburnishing parameters are fed into a computer control system thatcooperates with a burnishing apparatus (such as a CNC system) forperforming the burnishing operation (step 212). Inspecting the treatedcomponent for surface irregularities (step 214) is performed afterburnishing.

It should be understood that the method of the subject invention willimprove the SCC and H₂S cracking resistance of metallic componentsformed of less expensive alloys to allow them to be used in chloride orsulfide corrosive environments such as Fuel Recovery Components andchemical plant applications, where they cannot currently be used. Itshould also be understood that the method of the subject invention maybe integrated into any existing production or repair processingplatform/delivery system to allow for processing of both new productioncomponents as well as repair/life extension of existing components. Itshould also be understood that the method of the subject invention maybe used on various components used in corrosive environments such asmost or all metallic components used in a fossil fuel recovery where anyenvironmentally assisted cracking is expected or may occur.

Referring to FIGS. 4, 6, and 7, a schematic cross-section of a surfaceportion 106 of a metallic component used in a corrosive environment 108,such as Fuel Recovery Components or components used in chemical plantapplications having an outer surface 116 and an inner surface 118 isshown (FIG. 6). In a preferred embodiment, the component is in the formof a pipe, tube, casing, coupling, or other similar component. Thesurface portion 106 of the component 108 has a compressive residualstress distribution 110 induced therein. The depth D of compression issuch that it exceeds any surface irregularities 120. In anotherpreferred embodiment, the surface portion 106 has less thanapproximately 5% cold work. In another preferred embodiment of theinvention, the residual stress distribution extends through the entirethickness D of the surface portion 106.

It should be understood that the component 108 can include a pluralityof surface portions 106 or that the entire component can include one ormore surface portions 106. It should also be understood that eachsurface portion can have its own unique compressive stress distributionor a plurality of residual stress distributions can be utilized.

It should be understood that one or both of the inside and outsidesurface portions can be treated to improve the surface finish andproduce a depth of compression is such that all or a majority of surfaceirregularities, such as flaws, corrosion pits, persistent slip bands,and the like that may operate as sites of crack initiation are confinedwithin the depth of the compressive layer. In another preferredembodiment, the depth of the residual stress distribution in the surfaceportion extends to a depth of at least to about 1 mm. In anotherpreferred embodiment of the invention, the residual stress in thesurface portion is of a magnitude and depth such that the sum ofresidual and applied stress at the surface and to a depth never exceedsthe threshold for SCC in the corrosive environment of the application orthe fatigue endurance limit of the material forming the portion of thecomponent.

It should now be apparent to one skilled in the art that the subjectinvention provides corrosive resistant components that can be used attheir full available strength due to improved properties that mitigateor prevent the deleterious effects of stress corrosion cracking andfatigue. Further, that the invention is a practical, inexpensive methodof introducing a relatively deep, stable layer of beneficial compressivestress along and into the surface of Fuel Recovery Components, as wellas components for use in a wide variety of chemical plant applications,that protects against or reduces SCC, fatigue, corrosion fatigue andrelated failure modes with an improved surface finish, and low coldworking.

While the methods and components described herein constitute preferredembodiments of the invention, it is to be understood that the inventionis not limited to the precise method and components, and that changesmay be made therein without departing from the scope of the inventionwhich is defined in the appended claims.

1. A method of improving the properties of a metallic component for usein a corrosive environment comprising the steps of: identifying at leastone portion of the component that is expected to be exposed to acorrosive environment; determining a desired compressive stressdistribution for said at least one portion of the component that isexpected to be exposed to a corrosive environment; inducing said desiredcompressive stress distribution within at least a portion of the surfaceof a component.
 2. The method of claim 1 wherein said desiredcompressive stress distribution having a magnitude, depth, and cold workeffective to mitigate SCC and H₂S cracking of the component during usein a corrosive environment.
 3. The method of claim 1 wherein thecorrosive environment is identified and wherein the desired compressivestress distribution is determined for the identified corrosiveenvironment.
 4. The method of claim 1, wherein said compressive stressdistribution is induced using a CNC or robotically controlled burnishingand effective to impart the desired compressive stresses in a controlledmanner.
 5. The method of claim 1 wherein the step of inducing thedesired compressive stress distribution includes using a hydraulicallysupported burnishing apparatus.
 6. The method of claim 1 wherein thestep of inducing the desired compressive stress distribution includesthe steps of determining burnishing parameters to provide a smoothsurface along the at least one portion of the surface of the componentsuch that surface irregularities that can become crack or corrosion pitinitiation sites are placed in compression
 7. The method of claim 6wherein said burnishing parameters include the smoothness of theburnishing member that will be used for inducing compressive stresswithin the at least one portion of the surface of the component, thediameter of the of the burnishing member, the force with which theburnishing member will be pressed against the at least one portion ofthe surface of the component, and the pattern of burnishing.
 8. Themethod of claim 1 further comprises the step of identifying the materialproperties of the component, the applied loads expected to be applied tothe component, the environment in which the component is expected tooperate, and the known causes Of failure for similar components.
 9. Themethod of claim 1 further comprising the step of enhancing thesmoothness of the surface along the at least a portion of the surface ofa component such that surface irregularities that can become crack orcorrosion pit initiation sites are reduced or eliminated.
 10. The methodof claim 1 wherein the desired compressive stress has a magnitude anddepth of compression that extends to a depth of at least nominally ofabout 0.5 mm such that the sum of residual and applied stress neverexceeds the threshold for SCC in the corrosive environment of theapplication or the fatigue endurance limit of the material.
 11. Themethod of claim 1 wherein the induced desired compressive stress has adepth of compression that incorporates a majority of surfaceirregularities along the at least one portion of the surface of thecomponent.
 12. The method of claim 1 wherein the desired compressivestress distribution has a depth of penetration that penetrates entirelythrough the at least a portion of the surface of a component.
 13. Themethod of claim 1 wherein the compressive stress distribution within atleast a portion of the surface of the component has a depth that exceedsthe depth of any surface irregularities.
 14. A component for use in acorrosive environment comprising: at least one portion of the componenthaving a metallic surface; a compressive stress distribution within saidsurface; wherein the depth of said compressive stress distribution issuch that it exceeds a majority of surface irregularities along saidsurface and having an amount of cold work induced within said surfacethat is less than the amount necessary to damage the crystallinestructure along said surface and to create slip bands, dislocations, andtwinning such that said surface is more susceptible to stress corrosion.15. The metallic component of claim 14 wherein the said surface has adepth of compression is at least about 1 mm.
 16. The metallic componentof claim 12 wherein said stress distribution has a magnitude and depthof compression at least as great as the sum of any residual and appliedstress anticipated within said at least one portion.
 17. The metalliccomponent of claim 12 wherein said stress distribution has a depth ofcompression that does not exceed the threshold for SCC in the expectedcorrosive environment of the application of the component.
 18. Themetallic component of claim 12 wherein said at least one portion has adepth and said compressive stress distribution penetrates through theentire depth of the at least one portion.